1. Field of the Disclosure
The present disclosure generally relates to threaded connections having pipe dope. More particularly, the present disclosure relates to threaded connections having free volume for evacuation of pipe dope. More particularly still, the present disclosure relates to methods and apparatus to provide clearance gaps along an engaged thread area to result in a target free volume for the evacuation of pipe dope in large diameter wedge-threaded connections.
2. Background Art
Casing joints, liners, and other oilfield tubulars are frequently used to drill, complete, and produce wells. For example, casing joints may be placed in a wellbore to stabilize and protect a formation against high wellbore pressures (e.g., wellbore pressures that exceed a formation pressure) that could otherwise damage the formation. Casing joints are sections of pipe (e.g., steel or titanium), which may be coupled in an end-to-end manner by threaded connections, welded connections, or any other connection mechanisms known in the art. As such, connections are usually designed so that at least one seal is formed between an interior of the coupled casing joints and the annulus formed between exterior walls of the casing joints and the interior walls of the wellbore (i.e., the formation). The seals may be elastomeric (e.g., an o-ring seat), thread seals, metal-to-metal seals, or any other seals known to one of ordinary skill in the art.
It should be understood that certain terms are used herein as they would be conventionally understood, particularly where threaded tubular joints are connected in a vertical position along their central axes such as when making up a pipe string for lowering into a well bore. Typically, in a male-female threaded tubular connection, the male component of the connection is referred to as a “pin” member and the female component is called a “box” member. As used herein, “make-up” refers to engaging a pin member into a box member and threading the members together through torque and rotation. Further, the term “selected make-up” refers to the threading of a pin member and a box member together with a desired amount of torque or based on a relative position (axial or circumferential) of the pin member with respect to the box member. Furthermore, the term “box face” is understood to be the end of the box member facing outward from the box threads and the term “pin nose” is understood to be the end of the pin member facing outward from the threads of the connection. As such, upon make-up of a connection, the nose of the pin is stabbed or inserted into and past the face of the box.
Referring to the geometry of threads, the term “load flank” designates the side wall surface of a thread that faces away from the outer end of the respective pin or box member on which the thread is formed and supports the weight (i.e., tensile load) of the lower tubular member hanging in the well bore. Similarly, the term “stab flank” designates the side wall surface of the thread that faces toward the outer end of the respective pin or box member and supports forces compressing the joints toward each other such as the weight of the upper tubular member during the initial make-up of the joint or such as a force applied to push a lower tubular member against the bottom of a bore hole (i.e., compressive force).
One type of threaded connection commonly used to form a thread seal in oilfield tubulars is a wedge thread. In FIGS. 1A and 1B, a prior art connection 201 having a wedge thread is shown. “Wedge threads” are characterized by threads, regardless of a particular thread form, that increase in width (i.e., axial distance between load flanks 211 and 212 and stab flanks 213 and 214) in opposite directions on a pin member 203 and a box member 205. The rate at which the threads change in width along the connection is defined by a variable commonly known as a “wedge ratio.” As used herein, “wedge ratio,” although technically not a ratio, refers to the difference between the stab flank lead and the load flank lead, which causes the threads to vary width along the connection. Furthermore, as used herein, a thread “lead” refers to the differential distance between a component of a thread on consecutive threads. As such, the “stab lead” is the distance between stab flanks of consecutive thread pitches along the axial length of the connection. A detailed discussion of wedge ratios is provided in U.S. Pat. No. 6,206,436 issued to Mallis, and assigned to the assignee of the present invention. That patent is incorporated herein by reference in its entirety.
Wedge threads are extensively disclosed in U.S. Pat. No. RE 30,647 issued to Blose, U.S. Pat. No. RE 34,467 issued to Reeves, U.S. Pat. No. 4,703,954 issued to Ortloff, and U.S. Pat. No. 5,454,605 issued to Mott, all assigned to the assignee of the present invention and incorporated herein by reference in their entirety.
Referring Still to FIGS. 1A and 1B, on the pin member 201, a pin thread crest 239 is narrow towards the distal end of the pin member 201 while a box thread crest 243 is wide. Moving along an axis 200 (from right to left), the pin thread crest 239 widens while the box thread crest 243 narrows. As shown in FIGS. 1A and 1B, the threads are tapered, meaning that a pin thread 207 increases in diameter from beginning to end while a box thread 209 decreases in diameter in a complimentary manner. Having a thread taper may improve the ability to stab the pin member 203 into the box member 205 and distributes stress in the connection.
Generally, thread seals are difficult to achieve with non-wedge (i.e., free-running) threads. However, thread forms that are unable to form a wedge seal in a free-running configuration may create thread seals when used in a wedge thread configuration. As should be understood by one of ordinary skill, as wedge threads do not require any particular type or geometry of thread form, a variety of thread forms may be used. One example of a suitable thread form is a semi-dovetailed thread form disclosed in U.S. Pat. No. 5,360,239, issued to Klementich and incorporated herein by reference in its entirety. Another thread form includes a multi-faceted load flank or stab flank, as disclosed in U.S. Pat. No. 6,722,706, issued to Church and incorporated herein by reference in its entirety. Each of the above thread forms is considered to be a “trapped” thread form, meaning that at least a portion of the corresponding load flanks and/or corresponding stab flanks axially overlap. An open (i.e., not trapped) thread form with a generally rectangular shape is disclosed in U.S. Pat. No. 6,578,880, issued to Watts and incorporated herein by reference in its entirety. As such, the above thread forms (i.e., those of Klementich, Church, and Watts) are examples of thread forms that may be used with embodiments of the invention. Generally, open thread forms such as buttress or stub are not suitable for wedge threads, as they would impart a large radial force on the box member. However, a generally square thread form, such as that disclosed by Watts, or a trapped thread form, may be used, as they do not impart an outward radial force on the box member. As such, those having ordinary skill in the art will appreciate that the teachings contained herein are not limited to particular thread forms.
For wedge threads, a thread seal may be accomplished as a result of the contact pressure caused by interference over at least a portion of the connection 201 between the pin load flank 211 and the box load flank 212 and between the pin stab flank 213 and the box stab flank 214, which occurs when the connection 201 is made-up. Close proximity or interference between the roots 241 and 245 and crests 239 and 243 completes the thread seal when it occurs over at least a portion of where the flank interference occurs. Generally, higher pressure may be contained with increased interference between the roots and crests (“root/crest interference”) on the pin member 203 and the box member 205 and by increasing flank interference.
Referring again to FIGS. 1A and 1B, in wedge threads, a thread seal may be accomplished through contact pressure caused by interference that occurs at make-up over at least a portion of connection 201 between pin load flank 211 and box load flank 212 and between pin stab flank 213 and box stab flank 214. Close proximity or interference between roots 241 and 245 and crests 239 and 243 completes the thread seal when occurring proximate to such flank interference. Generally, higher pressures may be contained either by increasing interference between the roots and crests (“root/crest interference”) on pin member 203 and box member 205 or by increasing the aforementioned flank interference. The particular connection shown in FIG. 1 also includes a metal-to-metal seal that is accomplished by contact pressure between corresponding seal surfaces 204 and 206, respectively located on the pin member 203 and box member 205.
Although various wedge thread connections exist having positive-stop torque shoulders (e.g., Klementich, referenced above), wedge threads typically do not have torque shoulders, so their make-up is “indeterminate,” and, as a result, the relative position of the pin member and box member may vary more during make-up for a given torque range to be applied than for connections having a positive-stop torque shoulder. For wedge threads designed to have flank interference and root/crest interference at a selected make-up, the connection is designed such that both the flank interference and root/crest interference increase as the connection is made-up (i.e., an increase in torque increases flank interference and root/crest interference). For tapered wedge threads having root/crest clearance, the clearance decreases as the connection is made-up. Regardless of the design of the wedge thread, corresponding flanks come closer to each other (i.e., clearance decreases or interference increases) during make-up. Indeterminate make-up allows for the flank interference and root/crest interference to be increased by increasing the make-up torque on the connection. Thus, a wedge thread may be able to thread-seal higher pressures of gas and/or liquid by designing the connection to have more flank interference and/or root/crest interference or by increasing the make-up torque on the connection. However, increased interference and make-up torque may increase stress on the connection during make-up, which may lead to premature failure of the connection.
Before make-up, pipe dope is typically applied to both the pin member and the box member of a threaded connection. Pipe dope provides lubrication to aid the make-up of the connection and prevents galling to allow for the connection to be broken-out at a later time. In oilfield applications, the pipe dope typically contains metallic particles, such as copper, to prevent galling between the threads of the pin member and the box member. The metallic particles also help achieve a thread seal between wedge threads, which make-up on both the load and stab flanks.
Because of the close-fitting manner in which wedge threads make-up, as compared to a shouldered non-wedge thread connection, less pipe dope is required. Typically, the pipe dope is only applied to the pin thread of a wedge thread connection. The application of the pipe dope is also typically achieved with a brush instead of a large swab, as is typical of other non-wedge thread connections. When a wedge thread connection is made-up, excess pipe dope can become trapped between the pin thread and the box thread, which can cause false torque readings (leading to improper make-up) or potentially damage the connection. Many of the problems associated with the pipe dope can be mitigated by applying less pipe dope than non-wedge thread connections and controlling the speed at which the connection is made-up to allow for the pipe dope to squeeze out.
Actually damaging a connection as a result of pipe dope is rare, but is still a concern for operators. One scenario in which damage to the connection can occur is when the pipe dope is too viscous. This can occur in cold weather environments such as North Slope Alaska or the North Sea when the wrong pipe dope is used. For cold environments, pipe dope with lower metal content and reduced kinematic viscosity is supposed to be used. Kinematic viscosity is the ratio of the viscosity of a fluid to its density. Centistoke is a common unit for kinematic viscosity. A centistoke is the viscosity in centipoise divided by the liquid density at the same temperature. If the wrong pipe dope is used and the connection is made-up quickly, as is typical of a power frame used for making-up connections, the pipe dope can become trapped between the pin thread and the box thread, causing a high pressure build-up that expands the box member.
A more common scenario that can occur when making up a wedge thread connection is pipe stand-off. Pipe stand-off refers to the situation in which a connection gives a false torque reading that indicates the connection is fully made-up based on a make-up torque, but is not fully made-up based on the relative position of the pin member and the box member. One cause for pipe stand-off in wedge thread connections is hydraulic lock resulting from inadequate evacuation of pipe dope. The pressure build-up may then bleed off during use, risking accidental back-off of the connection or hydraulic leaks. Pipe stand-off is a particular concern for larger diameter threaded connections, such as those greater than or equal to about 9 inches in diameter (about 0.228 m). Dope evacuation is more difficult for larger diameter threaded connections because of the longer helical path for the pipe dope.
Furthermore, pipe stand-off may be particularly problematic in strings used at elevated downhole service temperatures (i.e., the temperature a tubular would expected to experience in service). Particularly, in high-temperature service (e.g., temperatures greater than 250° F., a steam-flood string, or a geothermal string), even a small amount of stand-off may be deleterious. For instance, if a made-up wedge connection with even a small amount of stand-off is deployed to a high-temperature well, the dope may flow out of the wedge thread connection and reduce the integrity of the thread seal.
Formerly, numerous attempts have been made to provide conduits for the evacuation of pipe dope either within or adjacent to the threads. For example, U.S. Pat. No. 3,822,902 issued to Mauer, et al, discloses a threaded connection for tubular goods including an externally threaded pin member, an internally threaded box member, a resilient seal ring positioned between the pin and box members to provide a fluid seal therein, and a passage formed in either the pin or box members for conducting thread lubricant or other liquid away from the seal ring as the pin and box members are screwed together. However, it should be understood that the axial passage taught of Mauer clearly does not contemplate a thread seal.
Next, U.S. Pat. No. 6,050,610 issued to Enderle et al, discloses a stress reduction groove for tubular connections including a box member and a pin member. The stress reduction groove of Enderle takes the form of a continuous groove formed in the thread of one of the pin and box members. The groove of Enderle extends from either the beginning or the end of the one thread to a point between the beginning and end of the one thread, thereby reducing the pressure that develops between the sealing surfaces during rotational make-up in a thread lubricant applied to the threads.
Similarly, U.S. Pat. No. 6,905,149, issued to DeLange et al, discloses lubricant escape passages formed in the threads used to connect one tubular body to another. The passages conduct trapped thread lubricant out of a threaded area to prevent the creation of high lubricant pressure that may damage or cause improper make up of the threaded connection. The passages may be grooves in the crests of the thread teeth and/or may be corner bevels on the thread teeth. When used with a wedge or other metal-to-metal thread engaging designs, the escape passages may be interrupted at some point intermediate their helical path to provide a pressure seal at the point of interruption. Relieving trapped lubricant from a wedge thread connection permits consistent final makeup positions that ensure optimal pressure sealing.
A wide range of pipe dopes are commercially available. Pipe dope is typically a proprietary formulation of lubricants) and particulates. In general, higher particulate concentrations result in more viscous pipe dope, which helps to provide a thread seal in wedge thread connections. The base grease is also largely determinative of the final kinematic viscosity of the pipe dope. One company providing pipe dope for threaded connections is JET-LUBE®, Inc. (Houston, Tex., USA). One type of pipe dope provided by JET-LUBE®, Inc. is KOPR-KOTE®, which contains less than 10 percent by weight of copper as the particulate additive. KOPR-KOTE® is provided in an alternative formulation for arctic use, as are several other JET-LUBE® formulations. Higher temperature pipe dopes (“thermal grade”) from JET-LUBE® utilize a petroleum oil with a kinematic viscosity of 414 to 506 centistokes at 40 degrees C. The “arctic grade” pipe dopes utilize a calcium base grease with a kinematic viscosity of about 20 to 24 centistokes at 40 degrees C., which is much lower than the thermal grade. Another pipe dope is JET-LUBE® NCS-30, which is specifically marketed for use with wedge thread connections. That pipe dope does not contain metallic particulates. Instead, JET-LUBE® NCS-30 uses a proprietary formulation of chemically inert fibers as the particulate additive. Also, JET-LUBE® NCS-30 uses a calcium base grease similar to the arctic grade compounds to provide reduced kinematic viscosity.
Although many of the problems with making-up a wedge thread are avoided by using a pipe dope with lower kinematic viscosity and/or reduced metal content, a disadvantage to such a pipe dope is reduced sealing ability in the wedge thread. The operating environment in the wellbore is much hotter than the surface, which allows for the pipe dope to flow more easily and not aid in maintaining the thread seal in the wedge thread. In general, the higher the kinematic viscosity of the pipe dope, the better the resulting thread seal in the wedge thread. Accordingly, it would be desirable to obtain better sealing capability for a connection with wedge threads by being able to use the better sealing forms of pipe dope regardless of the surface environment in which the connection will be made-up.